1. Field of the Invention
The invention relates to an electric power steering system.
2. Discussion of Background
Japanese Patent Application Publication No. 2006-213280 (JP 2006-213280 A) describes a rack assist-type electric power steering system (EPS) that includes a hollow shaft through which a rack shaft is passed and which is rotated by driving a motor. In the electric power steering system, the rotation of the hollow shaft is converted into a reciprocating linear motion of the rack shaft by a ball screw mechanism to apply an assist force to a steering system.
In such an EPS, usually, one end portion of the hollow shaft is rotatably supported by a bearing that receives a radial load and a thrust load, and the other end portion thereof is rotatably supported by a bearing that receives a radial load. The bearing that receives a thrust load and a radial load is disposed in a rack housing that accommodates the rack shaft, with its axial movement restricted, and the bearing that receives a radial load is disposed in the rack housing with its axial movement allowed. For example, an O-ring is disposed on the outer periphery of the bearing that receives a radial load, and this bearing is elastically supported in the radial direction. With this configuration, it is possible to avoid the situation where the bearing contacts the rack housing due to, for example, vibrations caused when a vehicle travels and abnormal noise is thus generated (see FIG. 2 of JP 2006-213280 A).
In the above-described EPS, in a steering within a range of small steering angles before the start of application of assist force, the hollow shaft is not driven by a motor, and hence the rack shaft is moved in its axial direction while the hollow shaft is rotated by a steering force applied by a driver via the ball screw mechanism. That is, at the start of the steering, the ball screw mechanism is actuated by the axial movement of the rack shaft to rotate the hollow shaft, and hence the driver needs to apply a relatively large steering force.
Therefore, as shown in, for example, FIG. 7, elastic members 72 are arranged at respective axial ends of an outer ring 71b of a bearing 71 which receives the above-described thrust and radial loads so that the bearing 71 is elastically supported relative to a rack housing 73 in the axial direction. In addition, O-rings 74 are arranged on the outer periphery of the bearing 71 so that the bearing 71 is elastically supported in the radial direction. Due to elastic deformation of the elastic members 72 and the O-rings 74, a hollow shaft 75 and the bearing 71 are allowed to move together relative to the rack housing 73 in the axial direction. As a result, a rack shaft 76 is allowed to move only by a small amount in the axial direction without rotating the hollow shaft 75. Consequently, it is possible to improve the steering feel at the start of the steering.
However, in the configuration shown in FIG. 7, because the outer periphery of each of both ends of the hollow shaft 75 is supported via the O-rings 74 made of a rubber material, the stiffness for radially supporting the hollow shaft 75 is low. As a result, the hollow shaft 75 easily rattles. As a result, when a steering wheel is operated, the bearing 71 tilts together with the hollow shaft 75 and the bearing 71 is pushed against and caught on the inner peripheral face of the rack housing 73 in some cases. In this case, if the steering wheel is returned, the bearing 71 is removed from the inner peripheral face of the rack housing 73 and returns to its original posture. At this time, the actual steering force instantaneously falls below a steering force required to reciprocate the rack shaft 76, which may cause deterioration of the steering feel. In the case where the hollow shaft 75 constitutes a rotor of a motor, if the hollow shaft 75 rattles, a gap between the hollow shaft 75 and a stator fixed to the inner periphery of the rack housing 73 changes. This may cause, for example, an increase in togging torque.